Transducers for generating wide frequency band acoustic energy in a borehole environment can be particularly useful in the acoustic investigation of the surrounding earth formation. Various transducers have been proposed as in, for example, the U.S. Pat. No. 3,496,533 to Semmelink in which a plurality of transducers are shown vertically stacked to achieve a desired acoustic beam steering capability of a receiver. A broad bandwidth is achieved by operating the receivers well below their resonance.
A segmented acoustic transducer is described in the U.S. Pat. No. 3,177,382 to Green. In the U.S. Pat. No. 4,004,267 to Mayne a technique is described for exploring a subsurface formation with acoustic pulses which individually are of narrow bandwidth but are spaced in frequency over a desired frequency spectrum. The frequency spacing may be in a geometric progression or in an arithmetic progression.
It can be particularly desirable in certain acoustic investigations of an earth formation from a borehole with a multiple receiver tool to use an acoustic transmitter capable of generating an acoustic pulse of relatively broad bandwidth. For example, when multiple receiver waveforms are used in a multifold correlation technique such as described in the U.S. Pat. No. 4,131,875 to Ingram, a broad bandwidth acoustic signal source provides a more narrow peak in the correlation function with low amplitude side lobes. A broad bandwidth acoustic signal source thus improves the ability to resolve significant acoustic waves such as compressional, shear and casing waves.